Processes for delignifying or bleaching pulp generally include at least one step of treating the pulp with chlorine dioxide. Also chlorine is a common pulp bleaching agent and even if the bleach plant does not include a chlorine step the chlorine dioxide often contains small amounts of chlorine as an impurity. Waste gases from bleach plants often contain low concentrations of chlorine dioxide as well as of chlorine. For environmental reasons both chlorine dioxide and chlorine must be destroyed or removed, and this is commonly done by scrubbing with different media. Many reactants are efficient for chlorine, for example caustic, but it is hard to find an inexpensive scrubbing media that is effective for both chlorine and chlorine dioxide without causing operational problems such as precipitation of solids or formation of other unwanted by-products.
Chemical Abstracts 94(4):17082, abstract of JP laid open patent application, publ. no. 55098965, discloses treatment of waste gases from textile or wood bleaching with sodium hydroxide in the presence of hydrogen peroxide and sodium silicate for conversion of chlorine dioxide to sodium chlorite.
WO 94/02680 discloses a process of removing color or chlorinated organic compounds from bleach plant effluents by utilizing ultraviolet light and oxygen, ozone, hydrogen peroxide or chlorine dioxide.
However, there is a need for an efficient process of removing or destroying chlorine dioxide and preferably also chlorine in waste gases which does not suffer from operational problems or high costs for chemicals used.